1. Field of the Invention
This invention relates to a semiconductor device having an air-bridge lead structure, and more particularly to a semiconductor device having an improved air-bridge lead structure which has a higher mechanical strength and a lower electric resistance with smaller electric capacitance than conventional air-bridge lead structures.
2. Description of the Prior Art
In a high-speed performance IC having a GaAs substrate and Schottky gate-type FETs formed thereon, the signal transfer delay at inner lead portions must be greatly reduced. To achieve this, the capacitance between the leads has been reduced by interposing therebetween substances having lower dielectric constants. More specifically, a so-called air-bridge lead structure has been developed in which a vacuum is formed between the leads or appropriate gases are contained between the leads whereby the leads are electrically insulated.
FIG. 3A is a cross-sectional view illustrating a conventional air-bridge lead structure. In FIG. 3A, first through fourth electrodes 2a through 2d are formed on the surface of a semiconductor substrate 1. An air-bridge lead 3 is formed crossing over the second and third electrodes 2b and 2c so as to connect the first electrode 2a to the fourth electrode 2d. Assume that the distance between the first and fourth electrodes 2a and 2d is relatively large. In this case, the air-bridge lead 3 might fall down by its own weight or by an external impact, as indicated by the dotted line shown in FIG. 3A. In general, the air-bridge lead 23 is made of materials such as Al, Au, Cu and the like which have a lower specific resistance.
However, these metals have lower rigidity, and thus the falling down of the air-bridge lead 23 inevitably occurs. If the air-bridge lead 23 is made of one of the metals such as W, Mo and the like, having a higher rigidity, such mechanical failures could be effectively suppressed. However, these metals have specific electric resistances much higher than those of metals such as Al, Au, Cu and the like. Thus, metals of W, Mo and the like are not suitable for practical applications. Assume that an air-bridge lead made of W is designed to have the same electric resistance per unit length as that of an air-bridge lead made of Au. In this case, the cross-sectional area of the former must be much greater than that of the latter. This inevitably increases the surface area of the air-bridge lead made of W. Thus, the electric capacitance around the air-bridge lead made of W inevitably increases. As a result, the signal transfer delay of the lead portions, which is determined depending on the product of RC (R=resistance, and C=capacitance), also increases.
FIG. 3B shows another conventional technique to solve the above-described disadvantage. Specifically, a redundant fifth electrode 2e which is electrically insignificant is formed on the surface of the substrate 1 at substantially the middle between the first and fourth electrodes 2a and 2d. A support 4 is connected between the air-bridge lead 3 and the fifth electrode 2e so as to prevent the falling down of the air-bridge lead 3. However, the air-bridge lead structure shown in FIG. 3B still has the following disadvantages. First, assume that metals such as Al, Au, Cu and the like having lower specific resistance and lower rigidity are employed to form the air-bridge lead. In this case, plural redundant electrodes and supports similar to the fifth electrode 2e and the support 4 must be provided at intervals of about 100 .mu.m. This degrades the integration density of ICs and prevents the miniaturization of semiconductor devices. Second, when heat is carried to the air-bridge lead made of one of metals such as Al, Au, Cu and the like, the lead is softend. Further, if the lead is left at a temperature of about 200.degree. C. or more for a long time, the lead will fall down.
As described above, in the conventional air-bridge lead structure, metals such as Al, Au, Cu and the like having lower specific electric resistance and lower rigidity have been employed to form the air-bridge lead. Thus, the falling down of the air-bridge lead inevitably arises when the internal temperature of the IC rises. On the other hand, metals such as W, Mo and the like having higher rigidity (higher Young's modulus) could be employed to enhance the mechanical strength of the air-bridge lead structure. However, these metals are not suitable for practical application because of their higher specific electric resistances. Further, redundant electrodes which are electrically insignificant have been formed on the substrate in order to support the air-bridge lead. However, these supports and redundant electrodes must be formed at intervals of about 100 .mu.m to achieve the desirable mechanical strength of the air-bridge lead structure. Such redundant electrodes are disadvantageous to the miniaturization of ICs.